1. Field of the Invention
The present invention relates to a metal cathode for an electron tube, and more particularly, to a thermoelectron emissive metal cathode which has high electron-emitting performance and a life span sufficiently improved for use in electron tubes, such as Braun tubes, camera tubes, and high frequency magnetron tubes.
2. Description of the Related Art
A conventional thermoelectron emissive cathode for an electron tube uses an oxide cathode. The oxide cathode includes an electron emissive oxide layer obtained by conversion of a ternary or binary carbonate, preferably (Ba, Sr, Ca)CO3 or (Ba, Sr)CO3 disposed on a base metal, mainly composed of Ni, with a slight amount of a reducing agent, such as Mg and Si. Since such an oxide cathode has a low work function, it has an advantage of relatively low operating temperature (700-800xc2x0 C.). However, since the electron-emitting performance of the oxide cathode is limited, it is difficult to provide a current density exceeding 1 A/cm2. When electron emission density is increased, the raw material is evaporated or melted due to Joule heating and the cathode deteriorates, since the oxide cathode is a semiconductor, and develops a large electrical resistance. Moreover, a resistive layer may be formed between the metal base and the oxide layer due to prolonged operation, which shortens the life span of the cathode.
In addition, since an oxide cathode is fragile and has poor adhesion to a base metal on which the oxide cathode is mounted, the life span of a cathode-ray device using the cathode is limited. For example, when only one of the three oxide cathodes of a color cathode-ray tube fails, the expensive device must be replaced.
Therefore, attempts to produce a high performance metal cathode, without the drawbacks of an oxide cathode, for use in a cathode-ray device have been made, but have met with only limited success.
FIG. 1 shows a conventional structure of a metal cathode. The metal cathode includes-an emitter 11 for emitting electrons and that is bonded to a sleeve 12 by laser welding or diffusion bonding. Since the metal cathode operates at a high temperature, 1100xc2x0 C. or higher, the sleeve 12 is usually Mo which has excellent mechanical and chemical characteristics at high temperature. However, the Mo of the sleeve 12 diffuses and moves to the surface of the emitter 11 during the operation of the metal cathode. As the amount of Mo on the surface of the emitter 11 increases, the work function (2.2 eV) of the metal cathode continuously increases due to the high work function of Mo. Consequently, the electron-emitting performance declines and life span of the cathode is shortened.
To overcome these problems of oxide cathodes and metal cathodes, various types of metal cathodes have been proposed. For example, it is known that a metal cathode based on lanthanum hexaboride (LaB6) has better strength and higher electron-emitting performance than an oxide cathode. A single crystal cathode of hexaboride can provide high current density, up to 10 A/cm2. However, since the life span of an LaB6 cathode is short, the LaB6 cathode has been used for vacuum electronic devices in which a cathode unit can be conveniently replaced. The short life span of an LaB6 cathode is caused by its high reactivity with the components of a heater. For example, when LaB6 contacts a W heater, a fragile compound is formed as a result of a chemical reaction.
U.S. Pat. No. 4,137,476 discloses a cathode including a barrier material between LaB6 and the body of a heater in order to remove the possibility of a reaction. However, the manufacturing cost of this structure is considerably increased over the LaB6 cathode, without a considerable improvement in the life span of the cathode.
USSR Patent No. 970,159 discloses a metal cathode formed by adding an alkaline earth metal to the main element, chosen from the platinum group of metals, improving the thermoelectron emissive characteristic and raising the secondary electron emission coefficient.
USSR Patent No. 1,365,948 discloses a metal cathode formed by adding a refractory metal to a metal alloy cathode, including an element of the platinum group metals and an alkaline earth metal, improving electron-emitting performance, improving shape stability and processibility at high temperature, and reducing cost.
In USSR Patent No. 1,975,520, an alkaline metal is added to a metal alloy including an element of the platinum group metals and an alkaline earth metal in order to decrease the operating temperature of the metal alloy and to raise the secondary electron emission coefficient.
However, none of these patents discloses a way to overcome the problem of diffusion of Mo into an emitter of a metal cathode.
To solve the above problems, it is an object of the present invention to provide a metal cathode in which the Mo component of a sleeve is prevented from diffusing into an emitter in order to avoid an increase in work function so that the metal cathode has better electron-emitting performance and a longer life span than an existing oxide cathode or metal cathode for a large scale and high definition electron tube.
Accordingly, to achieve the above object of the invention, there is provided an indirectly heated metal cathode for an electron tube, comprising a metal sleeve including Mo; a metal emitter disposed on the sleeve, the metal emitter containing at least one of Pt and Pd as a main component; and a buffer layer between the metal sleeve and the metal emitter. The sleeve is preferably an Moxe2x80x94Re alloy. The metal emitter is preferably an alloy comprising 85 to 99.5% by weight of at least one of Pt and Pd and 0.5 to 15% by weight of at least one of Ba, Ca, and Sr.
Preferably the buffer layer comprises at least one element selected from the group consisting of W, Hf, Ir, Ru, Zr, Nb, V, and Rh, more preferably Hf or W. The thickness of the buffer layer is 0.5-100 xcexcm, preferably 0.5-20 xcexcm, more preferably 3-10 xcexcm, and most preferably 5 xcexcm.
In addition, the area of the buffer layer may be the same as that of the metal emitter.